We build tools that read and write Excel files (open source library: https://github.com/sheetjs/js-xlsx) There are plenty of very difficult problems involving ill-specified aspects of the various file formats and errors in specifications, but it is largely a matter of grinding and finding files in the wild that capture the behavior you want to understand. Those are "difficult" in the sense that people still get these things wrong (related: recently a bug in the Oracle SmartView corrupted US Census XLS exports, which boiled down to an issue in calculating string lengths with special characters) but they don't feel difficult since most of the work didn't involve any really clever insights.
IMHO the hardest problem is now fairly straightforward: How do you enable people to test against confidential files? The solution involves running the entire process in the web browser using FileReader API: https://developer.mozilla.org/en-US/docs/Web/API/FileReader , and that is an obvious technical solution in 2017 but few thought it was even possible when we started.